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The photovoltaic eye. A new concept for very high photovoltaicefficiency

Objective

To develop and demonstrate the operation of a efficiencies above 30% of a photovoltaic converter based in the combination of silicon and gallium arsenide cells, located inside a light trapping cavity, and operates under concentrated sunlight. Heat removal and current extraction are also considered. The device is intended for cost competitive mass production of photovoltaic electricity.
Gallium arsenide photovoltaic cells of efficiency 23.9% at 172 suns and 22.7% at 624 suns have been fabricated. Silicon cells for photons with lower energy than the gallium arsenide band gap were obtained using a low doped base. The cells were assembled in a pair of cavities. An external filter was used to transmit photons in the range 400 to 900 nm to the gallium arsenide cell and reflect the remainder to the silicon. The gallium arsenide was tilted to the optical axis of the incident beam. Sunlight was collected with a spherical telescope mirror. The assembly was installed in an equatorial axis sun tracker, powered by a synchronous mains motor. Tracking was very accurate.

Maximum efficiency obtained was 29.4% with a power input of 1.66 W, 1.14 W incident on the gallium arsenide and 0.52 W incident on the silicon cell. The effective concentration was 161 for the gallium arsenide and 4.1 for the silicon. At maximum concentration (input 5.94 W) the efficiency was 28.5%.

A systematic study on the various soldering techniques was carried out to find an appropriate technique for large area void free soldering. Best results were obtained with a lead, tin and silver mixture (Pb Sn5 Ag2.5) and a tin, copper and indium mixture (Sn Cu3 In0r5) in a gas mixture of nitrogen with 5% hydrogen.

A heat transfer mounting for solar cells based on a copper coated ceramic was designed and tested. The soldering quality of solar cell structures to the ceramic plates was tested by X-ray and ultrasonic methods. The technique proved adequate.

A development programme of gallium arsenide and silicon concentrator solar cells was implemented. Gallium arsenide cells were fabricated which exhibited efficiencies up to 25% at a concentration ratio of 120.

A cooling system has been designed to remove the excess heat from photovoltaic cells and keep them as close to ambient temperature as possible. A mathematical model of a heat pipe cooling system was constructed to test the design, and a twelfth scale experimental rig was built. The heat load to the cooling system was supplied by an electrical heater to simulate the heat gain by the photovoltaic cells. 2 designs of high efficiency finned pipes were used, one with looped fins and the other with loops of metal strip.

The mathematical model conditions were defined based on an allowable temperature difference between the ambient air and the effluent air. The air flow rate required to remove the heat collected by the cells was then calculated. Once the air flow and the cross sectional area for heat flow were known, the air velocity and the Reynolds number could be calculated. From the Reynolds number, appropriate correlations gave the friction factor for the flow and the Nusselt number. These figures determined the pressure drop across the flow and the heat transfer coefficient, thus determining the power absorbed by the fan and the actual temperature of the heat pipe. The model predicted the experimental result well. For an air flow of about 24 cubic metres per hour, a block temperature of 88 C was predicted, and the experimental result was 90 C.
The photovoltaic eye is a novel device based on the combination of cells of several bandgaps located inside a light confining cavity. They make use of two principles: the increase of efficiency produced when cells of several bandgaps are illuminated with photons of energy above their bandgap, but close to it, and the increase of absorption, and therefore of efficiency, caused when the cells are located inside a light confining cavity.

It is well known that a strong absorber can be made with a sphere internally covered with an absorber much poorer. The reason for it is the occasion produced in such a cavity, for additional incidence of the photons reflected by the poor absorber covering the cavity walls. Conceptually such a spherical cavity can be covered with solar cells that does not require to absorb the light very strongly. In this way dense metal grids are allowed allowing high efficient concentration operation, and the requirements for good antireflection coatings can be less stringent, so allowing, possibly better surface passivation.

A drawback of the classical integrating spheres is that they have a small entry aperture area, as compared with the cell area, and thus a high concentration is required to operate at reasonable concentrations on the cells. This is avoided in this project by the development of cavities based on a new principle: the angular spatial restriction to the escaping light. If the escaping light is not allowed to leave the cavity in all directions then the entry aperture can be larger.

Thus in this project a cavity of such nature will be developed able to accommodate Gas and Si cells, in such a way that the light will first fall on the Gaas cells, so being filtered, and the low energy photons will be reflected by a back mirror located in the rear face of these cells and will fall on the Si cells. Most of the light reflected by these Si cells will fall again on them. This is a very interesting feature as the Si is rather transparent to the light, and more so to the long wavelengths reflected by the Gas cells. The multiple pass of the light by the same Si cells will enhance greatly this absorption.

As the goal of the project is a prototype, aspects regarding the way of producing concentrated illumination, removing the excess heat an assembling the different elements are taken into account.

Coordinator

UNIVERSIDAD POLITECNICA DE MADRID
Address
Ciudad Universitaria - Etsi Telecomunicacion
28040 Madrid
Spain

Participants (2)

FRAUNHOFER GESELLSCHAFT ZUR FORDERUNG DER ANGEWANDTEN FORSCHUNG E.V.*
Germany
Address
Heidenhofstrasse 2
79110 Freiburg (In Breisgau)
UNIVERSITY OF READING
United Kingdom
Address
Whiteknights, Whiteknights House, Palmer Building
RG6 6AH Reading / Silchester